Date of Thesis

Spring 2020

Description

Thick successions of river deposits accumulated in the north-central Pennsylvania region of the Appalachian foreland basin during Late Devonian time (~380-360 Ma). The properties and morphologies of these paleorivers are not well characterized. Latest Devonian tectonic, climatic, and eustatic controls on river dynamics and basin infilling also remain unclear. This study assesses the sedimentology, facies architecture, paleochannel depths, and grain size of a 133 m thick section of fluvial strata exposed across two outcrops, Blossburg South (older) and Blossburg West (younger), mapped as lower Huntley Mountain Formation near Blossburg, Pennsylvania. Field-based lithofacies observations, high-resolution panoramic photography, terrestrial lidar scanning, and laser particle size analysis were used to build a stratigraphic column, map fluvial architecture, and estimate average paleoriver gradient.

Channel facies primarily consist of cross- and horizontally-stratified, fine-grained single- to multistory sand bodies with scours and lateral accretion surfaces. Proximal floodplain facies consist of gray thinly-bedded crevasse splay sandstones and massive levee siltstones and are more abundant upsection. Distal floodplain facies chiefly consist of red relict-bedded and fissile mudstone paleosols (with rootlets, slickensides, caliche nodules) and are more abundant downsection. Paleochannel depths (0.9-2.7 m) and bar deposit grain sizes (median diameters 142-221 μm) increase upsection. Estimated average landscape/river paleoslope for the latest Devonian alluvial plain near the studied Blossburg locality (0.54 x10-4 – 5.2 x10-4) does not exhibit any consistent change over the studied interval. The two outcrops are interpreted as a succession of suspended- to mixed-load meandering rivers within mobile channel-belts due to the predominance of lateral accretion surfaces, low median grain size (fine-grained sand range), and low paleoslope values (<10-3).

Several variables are explored to explain the observed stratigraphic changes. An upsection increase in channel facies proportion, channel depth, and grain size could be generated by a prograding distributive fluvial system, which has already been proposed to explain Late Devonian alluvial plain sedimentation. Evidence for rapid uplift, high sediment supply, and decreasing basin subsidence during Famennian time suggests a decrease in the ratio of accommodation-creation to sedimentation, which is consistent with system progradation. Biostratigraphic data interpret the age of Blossburg West strata to within the LN biozone (361-359 Ma), which is contemporaneous with the Hangenberg global cooling interval and an advance of lowland glaciers in the Appalachian foreland basin. Pro-glacial outwash and increased precipitation could potentially have influenced the deepening and sediment coarsening of Blossburg paleorivers, which subsequently would lead to an increasing channel facies proportion and amalgamation. However, improved relative age correlations between glacigenic strata in eastern Pennsylvania and fluvial strata in north-central Pennsylvania are necessary to confirm glacial influences on landscape/channel processes at the Blossburg locality.

Keywords

Sedimentology, Appalachian, Fluvial, Huntley Mountain, Late Devonian, Geology

Access Type

Honors Thesis

Degree Type

Bachelor of Science

Major

Geology & Environmental Geosciences

First Advisor

Ellen Chamberlin

Second Advisor

Jeffrey Trop

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